Remove repeated line in system updates and rollback

Remove a line that was repeated by mistake in system updates and rollback.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

content/designs/system-updates-and-rollback.md

@@ -806,7 +806,6 @@ this provides the new content that will be used for the read-only portions of
 the rootfs, but does not modify the read-write parts.
 To handle the upgrade of system settings stored in `/etc`, a copy of its
 default values are kept in `/usr/etc` which is updated with the new commit.
-default values is kept in `/usr/etc` which is updated with the new commit.
 Thanks to this information `OSTree` can detect the files that have been changed
 and apply a 3-way merge, to update the `/etc`.